The ability to isolate DNA and clone such isolated DNA into bacterial plasmids has greatly expanded the approaches available to make viral vaccines. The methods used to make the present invention involve modifying cloned DNA sequences from various viral pathogens of animals, by insertions, deletions, single or multiple base changes, and subsequent insertions of these modified sequences into the genome of the virus. One utility of the addition of a foreign sequence is achieved when the foreign sequence encodes a foreign protein that is expressed during viral infection of the animal. The resulting live virus may then be used in a vaccine to elicit an immune response in a host animal and provide protection to the animal against disease. A virus with these characteristics is referred to as a viral vector, because it becomes a living vector that will carry and express the foreign protein in the host animal. In effect it becomes an elaborate delivery system for the foreign protein(s).
More specifically, the present invention relates to the use of herpesvirus of turkeys (HVT) as a viral vector for vaccination of birds against disease. The group of herpesviruses comprise various pathogenic agents that infect and cause disease in a number of target species: swine, cattle, chickens, horses, dogs, cats, etc. Each herpesvirus is specific for its host species, but they are all related in the structure of their genomes, their mode of replication, and to some extent in the pathology they cause in the host animal and in the mechanism of the host immune response to the virus infection.
The application of recombinant DNA techniques to animal viruses has a relatively recent history. The first viruses to be engineered have been those with the smallest genomes. In the case of the papovaviruses, because these viruses are so small and cannot accommodate much extra DNA, their use in genetic engineering has been as defective replicons. Foreign gene expression from these viruses requires a wild-type helper virus and is limited to cell culture systems. For adenoviruses, there is a small amount of nonessential DNA that can be replaced by foreign sequences. The only foreign DNA that seems to have been expressed in adenoviruses are the T-antigen genes from papovaviruses (Mansour, et al., Proc. Natl. Acad. Sci. US, 1985; Thummel, et al., Cell, 1983; Scolnick, et al., Cell, 1981; Thummel, et al., Cell, 1981), and the herpes simplex virus (HSV) thymidine kinase gene (Haj-Ahmed and Graham, J. of Virology, 1986). These publications do not identify the nonessential regions in HVT wherein foreign DNA may be inserted, nor do they teach how to achieve the expression of foreign genes in HVT, e.g., which promoter sequence and termination sequence to use.
Another group of viruses that have been engineered are the poxviruses. One member of this group, vaccinia, has been the subject of much research on foreign gene expression. Poxviruses are large DNA-containing viruses that replicate in the cytoplasm of the infected cell. They have a structure that is unique in that they do not contain any capsid that is based upon icosahedral symmetry or helical symmetry. The poxviruses are most likely to have evolved from bacterial-like microorganisms through the loss of function and degeneration. In part due to this uniqueness, the advances made in the genetic engineering of poxviruses cannot be directly extrapolated to other viral systems, including herpesviruses and HVT. Vaccinia recombinant virus constructs have been made in a number of laboratories that express the following inserted foreign genes: HSV thymidine kinase gene (Mackett, et al., Proc. Natl. Acad. Sci. USA, 1982; Panicali and Paoletti, Proc. Natl. Acad. Sci. USA, 1982, hepatitis B surface antigen (Paoletti, et al., Proc. Natl. Acad. Sci. USA, 1984; Smith et al., Nature, 1983), HSV glycoprotein D gene, infaluenzae hemagglutinin gene (Panicali, et al., Proc. Natl. Acad. Sci. USA, 1983; Smith, et al., Proc. Natl. Acad. Sci. USA, 1983), malaria antigen gene (Smith, et al., Science, 1984, and vesicular stomatitis glycoprotein G gent (Mackett, et al., Science, 1986). The general overall features of vaccinia recombinant DNA work are similar to the techniques used for all the viruses, especially as they relate to the techniques in reference (Maniatis, et al., Molecular Cloning, 1982). However in detail, the vaccinia techniques are not applicable to herpesviruses and HVT. The utility of vaccinia as a vaccine vector is in question because of its close relationship to human smallpox and its known pathogenicity to humans. Thus, the use of the host-specific herpesvirus HVT is a better solution to vaccination of poultry.
Among the primate herpesviruses, only HSV of humans and, to a limited extent, herpes saimiri of monkeys have been engineered to contain foreign DNA sequences. The first use of recombinant DNA to manipulate HSV involved cloning a piece of DNA from the L-S junction region into the unique long region of HSV DNA, specifically into the thymidine kinase gene (Moccarski, et al., Cell, 1980). This insert was not a foreign piece of DNA, rather it was a naturally occurring piece of herpesvirus DNA that was duplicated at another place in the genome. This piece of DNA was not engineered to specifically express a protein, and thus this work does not involve expression of protein in herpesviruses. The next manipulation of HSV involved the creation of deletions in the virus genome by a combination of recombinant DNA techniques and thymidine kinase selection. Using this approach, the HSV alpha-22 gene has been deleted (Post, et al., Cell, 1981), and a 15,000 basepair sequence of DNA has been deleted from the internal repeat of HSV (Poffenberger, et al., Proc. Natl. Acad. Sci. USA, 1981).
The following cases involve insertion of genes that encode protein into herpesviruses: the insertion of HSV glycoprotein C into a naturally occurring deletion mutant of this gene in HSV (Gibson and Spear, J. of Virology, 1983); the insertion of glycoprotein D of HSV type 2 into HSV type 1 (Lee, et al., Proc. Natl. Acad. Sci. USA, 1982), with no manipulation of promoter sequences since the gene is not `foreign`; the insertion of hepatitis B surface antigen into HSV under the control of the HSV ICP4 promoter (Shih, et al., Proc. Natl. Acad. Sci. USA, 1984); and the insertion of bovine growth hormone into herpes saimiri virus with an SV40 promoter (the promoter did not work in this system and an endogenous upstream promoter served to transcribe the gene) (Desrosiers, et al., 1984). Two additional foreign genes (chicken ovalbumin gene and Epstein-Barr virus nuclear antigen) have been inserted into HSV (Arsenakis and Roizman, 1984), and glycoprotein X of pseudorabies virus has been inserted into HSV (Post, et al., 1985).
These cases of deletion or insertion of genes into herpesviruses demonstrate that it is possible to genetically engineer herpesvirus genomes by recombinant DNA techniques. The methods that have been used to insert genes involve homologous recombination between the viral DNA cloned in plasmids and purified viral DNA transfected into the same animal cell. However, the extent to which one can generalize the location of the deletion and the sites for insertion of foreign genes is not known from these previous studies.
One object of the present invention is a vaccine for Marek's disease. Marek's disease virus (MDV) is the causative agent of Marek's disease which encompasses fowl paralysis, a common lymphoproliferative disease of chickens. The disease occurs most commonly in young chickens between 2 and 5 months of age. The prominent clinical signs are progressive paralysis of one or more of the extremities, incoordination due to paralysis of legs, drooping of the limb due to wing involvement, and a lowered head position due to involvement of the neck muscles. In acute cases, severe depression may result. In the case of highly oncogenic strains, there is characteristic bursal and thymic atrophy. In addition, there are lymphoid tumors affecting the gonads, lungs, liver, spleen, kidney and thymus (Mohanty and Dutta, 1981).
Most chickens are vaccinated against MDV at one day of age to protect the bird against MDV for life. Prior to the present invention, the principal vaccination method for MDV involved using naturally occurring strains of turkey herpesvirus (HVT). It would be advantageous to incorporate other antigens into this vaccination at one day of age, but efforts to combine conventional vaccines have not proven satisfactory to date due to competition and immunosuppression between pathogens. The multivalent HVT-based vaccines engineered in this invention represent a novel way to simultaneously vaccinate against a number of different pathogens. For the first time, a recombinant HVT with a foreign gene inserted into a non-essential region of the HVT genome is disclosed.
The types of genetic engineering that have been performed on these herpesviruses consist of cloning parts of the virus DNA into plasmids in bacteria, reconstructuring the virus DNA while in the cloned state so that the DNA contains deletions of certain sequences, and furthermore adding foreign DNA sequences either in place of the deletions or at sites removed from the deletions.
A foreign gene of interest targeted for insertion into the genome of HVT may be obtained from any pathogenic organism of interest. Typically, the gene of interest will be derived from pathogens that in poultry cause diseases that have an economic impact on the poultry industry. The genes may be derived from organisms for which there are existing vaccines, and because of the novel advantages of the vectoring technology the HVT derived vaccines will be superior. Also, the gene of interest may be derived from pathogens for which there is currently no vaccine but where there is a requirement for control of the disease. Typically, the gene of interest encodes immunogenic polypeptides of the pathogen, and may represent surface proteins, secreted proteins and structural proteins.
A relevant avian pathogen that is a target for HVT vectoring is Infectious Laryngotracheitis virus (ILTV). ILTV is a member of the herpesviridiae family, and this pathogen causes an acute disease of chickens which is characterized by respiratory depression, gasping and expectoration of bloody exudate. Viral replication is limited to cells of the respiratory tract, where in the trachea the infection gives rise to tissue erosion and hemorrhage. In chickens, no drug has been effective in reducing the degree of lesion formation or in decreasing clinical signs. Vaccination of birds with various modified forms of the ILT virus derived by cell passage and/or tedious regimes of administration have conferred acceptable protection in susceptible chickens. Because of the degree of attenuation of current ILT vaccines care must be taken to assure that the correct level of virus is maintained; enough to provide protection, but not enough to cause disease in the flock.
An additional target for the HVT vectoring approach is Newcastle disease, an infectious, highly contagious and debilitating disease that is caused by the Newcastle disease virus (NDV). NDV is a single-stranded RNA virus of the paramyxovirus family. The various pathotypes of NDV (velogenic, mesogenic, lentogenic) differ with regard to the severity of the disease, the specificity and symptoms, but most types seem to infect the respiratory system and the nervous system. NDV primarily infects chickens, turkeys and other avian species. Historically vaccination has been used to prevent disease, but because of maternal antibody interferences, life-span of the bird and route of administration, the producer needs to adapt immunization protocols to fit specific needs.